1. Field of the Invention
This invention pertains to pulse oximetry, and more particularly to discrimination between valid and artifactual pulse waveforms obtained by pulse oximetry.
2. Prior Art
Commercially available pulse oximeters provide instantaneous in vivo measurements of arterial oxygenation by determining the color of blood between a light source and a photodetector. To distinguish between the two main types of hemoglobin, i.e., oxyhemoglobin and reduced hemoglobin, measurement of light absorption is carried out at two wavelengths using a light source that consists of two different light emitting diodes, one for red and the other for infrared light. With each cardiac cycle, there is cyclic light absorption by tissue beds. During diastole, absorption is a result of venous blood, tissue, bone and pigments (melanin, nail polish, etc.). During systole, light absorption is increased by influx of arterialized blood into the tissue bed. The oximeter determines the difference between background absorption during diastole and peak absorption during systole at both red and infrared wavelengths, as this difference corresponds to the absorption caused by arterialized blood. Since oxygen saturation determines the red:infrared light absorption ratio, differences in this ratio are used to compute the arterial oxygen saturation, a value derived empirically by the manufacturer's calibration curve. See, e.g. Schnapp, et al., Pulse Oximetry: Uses and Abuses, Chest 1990; 98: 1244-1250.
Accurate measurement of arterial oxygen saturation is highly dependent upon a valid pulse waveform, typically detected by a sensor disposed on an extremity or, in the case of adults, on the nose or ear. However, the pulse waveform can be distorted, rendering corresponding arterial oxygen saturation values invalid. For example, the pulse waveform can be distorted by vasoconstriction associated with hypotension or motion of the body site where the sensor is affixed. To verify the validity of the pulse waveform, some pulse oximeters display the pulse waveform in real time so the operator call judge whether a valid waveform is present. Others use the electrical amplitudes of the waveform as a guide to waveform validity and display a message if artifacts are present. For example, the amplitude of the varying component of the pulse waveform (AC) may be compared against the background light absorption (DC) or an arbitrary value. It has been suggested that the pulsatile strength is adequate and hence the pulse waveform valid when the AC/DC ratio exceeds 0.2%, but this standard has not been validated. At least one manufacturer of pulse oximeters employs an autogain program to flesh out low amplitude pulse waveforms for display when the AC/DC ratio falls below 0.2%. However, arterial oxygen saturation values at signal strengths below this threshold may still be valid provided the pulse waveform has a normal configuration. See, e.g. Wukitsch, et al., Pulse Oximetry: Analysis of Theory, Technology, and Practice, J. Clin. Monit. 1988; 4: 290-301.
At least one manufacturer of pulse oximeters has approached the motion artifact problem by synchronizing data acquisition by the pulse oximeter with the R wave of an EGG. However, the oximeter may synchronize with EGG artifacts caused by motion or shivering, resulting in erroneous readings. Furthermore, with this approach the pulse rate displayed by the oximeter is necessarily equal to the heart rate derived from the EGG, thereby eliminating concordance of pulse oximeter pulse rate with ECG heart rate as a validator of pulse oximetry data. See Alexander, et al., Principles of Pulse Oximetry: Theoretical and Practical Considerations. Anesth. Analg. 1989; 68: 368-376. This technique may also be subject to error because of the variability in operator and/or computer selection of peak to peak and trough to trough points of the pulse waveform, as well as inherent errors related to sampling rates, all of which affect the calculation of arterial saturation values.
U.S. Pat. No. 5,025,791 proposes detecting artifactual pulses by incorporating a motion detector in the pulse oximeter sensor. This, of course, requires the incorporation of additional components in the pulse oximeter sensor, as well as additional circuitry for processing the information derived from the motion sensor. U.S. Pat. No. 4,955,379 proposes eliminating artifactual pulse waveforms by employing a band pass filter.
Pulse oximeters have analog or digital outputs for values of arterial oxygen saturation as well as for pulse waveforms, and these outputs can be recorded and displayed on polygraph recorders and/or digital computers. These recordings can then be reviewed and correct values of arterial oxygen saturation recognized from the appearance of the pulse waveform, i.e. if the pulse waveform is correctly configured the corresponding arterial oxygen saturation value is accepted, whereas if the waveform is perceived as artifactual, the arterial oxygen saturation value is rejected. However, with lengthy recordings as in overnight studies in adults or babies, visual scanning is impractical due to time constraints, since as many as 20,000 to 80,000 or more pulse waveforms may be collected.
In view of the above, there is a need for confidently assessing the validity of arterial oxygen saturation measurements obtained with pulse oximetry, preferably in real time, and particularly for monitoring long term trends, as in critically ill patients.
It is accordingly an object of the present invention to provide a method and apparatus for validating arterial oxygen saturation measurements obtained with pulse oximetry.
It is another object of the invention to provide a method and apparatus for verifying arterial oxygen saturation measurements derived from pulse oximetry by verifying the validity of the pulse waveforms detected by a pulse oximetry sensor.
It is a further object of the present invention to provide a method and apparatus of the aforementioned type which discriminates between valid and invalid pulse waveforms on a real time basis.
It is yet a further object of the present invention to provide a method and apparatus of the aforementioned type which is highly accurate in its discrimination between valid and invalid pulse waveforms, even in the case of low amplitude pulse waveforms.